Since 1970 there has been an increasing demand for honeycomb cores that have structural integrity after operating for periods ranging upwards to 5000 hours in hot, cyclic corrosive atmospheres, such as found in the exhaust of spark-ignited or compression ignited internal combustion engines, e.g., diesel engines and turbines.
Honeycomb cores used in these applications can be coated with catalytic materials and used as catalytic converters. Such as disclosed in my copending application Ser. No. 830,698 filed Feb. 18, 1986, now U.S. Pat. No. 4,711,009, incorporated herein by reference. Alternately, such cores can be coated to resist hot, cyclic corrosion and can serve as diesel-engine particulate traps, recuperators or diffusers.
Metallic honeycomb cores are made either spirally wound or accordion folded. Some of the prior art cores are comprised of alternating flat metal and corrugated metal substrate layers. Alternately, the cores can be made of adjacent layers of corrugated metallic substrate of minimal thickness, e.g., 0.001" to 0.010" (thin metal) and containing a pattern such that nesting of the corrugations in adjacent layers does not take place. For example, a herringbone or sine wave pattern in the substrate will not nest with itself when the substrate is folded back and forth on itself. Furthermore, nesting will not take place when one of a pair of wave-pattern substrates is turned over or turned end for end and wound against the other one of the pair.
In the mass production of honeycomb cores, it is important that nesting does not take place, because if the adjacent corrugations nest together, then the overall size of the core is reduced, which leads to looseness of the core in the containment vessel. This subsequently leads to vibration of unsupported laminations. Vibration of laminations leads to failure of sections of the core and finally to catastrophic failure of the core as a whole.
Further, in the mass production of honeycomb cores it is essential to keep material usage at a minimum, because the substrate material is costly, especially in relation to the cost of the most commonly-used ceramic substrates. Twenty percent less substrate is needed for a given core size if the core construction consists of alternate layers of metallic substrate with patterned corrugations positioned between layers of similarly-formed corrugations but juxtapositioned by 180 degrees, so as not to nest, which is known as "mixed-flow cell construction" or a "mixed flow core".
Mixed-flow cell construction has the further advantage that greater contact is made with molecules of fluids as they pass by and are catalyzed by catalysts carried by the cell-walls in the core, in comparison with straight, annular cells. Particulate trapping is also enhanced with this mode of construction.
While nesting is not an issue with adjacent flat and corrugated substrate laminations, nesting can be a serious problem, for the reasons described above, in the case of mixed-flow cell construction. Nonetheless, mixed-flow construction has, on balance, so many advantages compared with annular cell construction that it is used increasingly for mass-produced honeycomb cores.
James R. Mondt has described a herringbone pattern for a recuperator in U.S. Pat. No. 3,183,963, issued May 18, 1965, "Matrix for Regenerative Heat Exchangers". Chapman has described a herringbone pattern in U.S. Pat. No. 4,318,888, "Wound Foil Structure", which when formed into a core will not nest. Cairns has described in U.S. Pat. No. 4,098,722, "Methods of Fabricating Bodies", a variable-pitch corrugation whereby adjacent faces will not nest.
The corrugated mixed-flow substrate manufacturing process, as well as the design of corrugation geometry and pattern, must be considered in production of honeycomb cores that are expected to endure the rigors of automotive field service.
The most practical means of manufacturing thin, corrugated substrate is to roll-form strips of metal foil through opposing intermeshing helical gears. The design of the teeth in the opposing gears dictates the corrugation-pattern, pitch and amplitude of the corrugations impressed in the substrate. The nature of the pattern in turn dictates the internal stresses in the foil substrate. As the substrate is pulled into the rotating, opposed gears, thinning of the substrate occurs wherever the substrate is in tension, or alternately thickening or bunching, where the substrate is in compression.
It is a principal object of this invention to describe continuouslyformed corrugation-patterns, for use in particle traps, in which there is a pattern-modal-relationship between adjacent laminations that is minimized. A pattern-modal-relationship is the relative degree of pattern-corrugations facing each other in adjacent layers. Pattern-modal-relationship is important, because the particle trap-efficiency of metal honeycomb cores used with diesel engines, can be improved substantially if patterns with a limited pattern-modelrelationship are used, being illustrated in the annexed drawings in FIGS. 1 and 2.
If the pattern-modal-relationship is minimized, then the carbon particles will be trapped broadly across the corrugated surfaces and trap efficiency will be increased. An optimized geometry for particulate traps is shown in FIGS. 1 and 2.